Variable equilibrate voltage circuit for paired digit lines

ABSTRACT

A method and circuit for rapidly equilibrating paired digit lines of a memory array of a dynamic random access memory device is described. The equilibrate circuit includes a bias-circuit coupled to sense amplifier circuitry for adjusting the equilibrate voltage during testing. A method is described for testing memory cell margin by adjusting the equilibrate voltage until an error is detected. The bias circuit is described as a pull-up transistor coupled to a common mode of a cross-coupled n-sense amplifier.

This application is a continuation of U.S. Ser. No. 08/977,757, filed Nov. 25, 1997, now U.S. Pat. No. 5,903,502.

FIELD OF THE INVENTION

The present invention relates to testing of semiconductor memory devices, such as dynamic random access memory devices, and in particular, the invention relates to a method and circuit for adjusting an equilibrate voltage of paired digit lines of a memory device.

BACKGROUND OF THE INVENTION

Semiconductor memory devices, such as dynamic random access memory (DRAM) devices typically store data as a logic one or zero. The data is represented as a charge stored on a capacitor. The charge is shared with a second capacitor, such as a digit line, to produce a resultant voltage. The voltage is then compared to a reference voltage to determine the correct data state. That is, a resultant voltage greater than the reference voltage is a one, and a resultant voltage less than the reference voltage is a zero.

Prior to any memory access cycle, for normal operation of the memory and during testing of the memory, paired digit lines are equilibrated to a common potential, typically one-half the supply voltage Vcc. Memory devices include equilibration circuits for this purpose. The equilibration circuit typically comprises one or more transistors that are connected between the digit lines that form a pair of paired digit lines. These transistors are enabled by an equilibrate enable signal that is provided prior to the start of a memory access cycle. When enabled, the transistors short the paired digit lines together. In a differential sensing scheme, the digit lines are charged to opposite supply voltage potentials. Thus, the resultant equilibrate voltage is ½ Vcc.

When testing an integrated circuit memory device it is useful to test memory cell margins, or the amount of voltage a memory cell can move a digit line above an equilibrate voltage. As such, a memory cell with a small margin may result in erroneous data storage. This is particularly true if the memory cell is partially discharged prior to a data read operation. A separate bias voltage can be coupled to a digit line prior to reading data stored in the memory to stress cell margins. This technique is unattractive because of the relative slowness of changing the equilibrate voltage after the standard equilibrate operation is completed.

For the reasons stated above, and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method and circuit for rapidly equilibrating paired digit lines of a semiconductor memory to a variable voltage, such as during testing.

SUMMARY OF THE INVENTION

The present invention provides a circuit fabricated in an integrated circuit memory device for equilibrating paired digit lines of a memory. The circuit allows the digit lines to be quickly equilibrated to an equilibrate voltage which is different than an equilibrate voltage used during normal memory operations.

In accordance with one embodiment of the invention, there is provided a memory device comprising an array of memory cells arranged in rows and columns, a digit line pair located in the array, and a sense amplifier circuit coupled to the digit line pair for sensing and amplifying a differential voltage between the digit line pair. The memory device also comprises a bias circuit coupled to the sense amplifier circuit for providing a bias voltage to one digit line of the digit line pair during a test operation, and an equilibrate circuit coupled to the digit line pair for equilibrating the digit line pair to a common voltage.

In another embodiment, a Dynamic Random Access Memory comprises an array of memory cells arranged in rows and columns, first and second digit lines arranged as a digit line pair and located in the array, and a sense amplifier circuit coupled to the first and second digit lines for sensing and amplifying a differential voltage between the first and second digit lines. The sense amplifier comprises first and second cross coupled transistors. The first transistor having a drain coupled to the first digit line, a gate coupled to the second digit line, and a source coupled to a source of the second transistor. The second transistor having a drain coupled to the second digit line, and a gate coupled to the first digit line. A pull-up transistor is coupled between the source of the second transistor and a bias voltage, the pull-up transistor controlled by an activate signal coupled to its gate. An equilibrate circuit is coupled to the digit line pair for equilibrating the first and second digit lines to a common equilibrate voltage.

In yet another embodiment, a method of equilibrating a memory device during a test mode is described. The method comprises the steps of initiating a test mode, accessing a memory cell for either reading from or writing data to the memory cell, and activating sense amplifier circuitry. The sense amplifier detects a differential voltage between complementary digit lines, and drives a first digit line to a high voltage and a second digit line to a low voltage. The method further comprises isolating the memory cell, activating a bias circuit to raise the second digit line voltage, and coupling the complementary digit lines together to equilibrate the first and second digit lines to a common test equilibrate voltage.

A method of testing memory cell margin in a memory device is also described. The method comprises the steps of equilibrating first and second digit lines to a first level, reading data stored in a memory cell, and restoring the data to a memory cell. The first and second digit lines are equilibrated to a second level which is greater than the first level via a bias circuit and sense amplifiers. The steps of retreading the data stored in the memory cell, and equilibrating the first and second digit lines to successively higher equilibrate voltages until a data read error is detected are also included in the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified representation of a semiconductor memory device incorporating the present invention;

FIG. 2 is a simplified representation of the memory array and a portion of the memory array access circuitry for the memory device of FIG. 1;

FIG. 3 is a schematic diagram of a memory cell;

FIG. 4 is a schematic diagram of the memory array of FIG. 1;

FIG. 5 is a timing diagram of a memory cell access operation;

FIG. 6 is a schematic diagram of a typical sense amplifier circuit;

FIG. 7 is a timing diagram of the operation of the sense amplifier circuit of FIG. 6;

FIG. 8 is a more detailed schematic of a portion of the array of FIG. 1;

FIG. 9 is timing diagram illustrating waveforms for the equilibration circuit of FIG. 8 provided by the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific preferred embodiment in which the invention may be practiced. The preferred embodiment is described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

FIG. 1 is a simplified representation of a semiconductor memory device 10 incorporating the present invention. In one embodiment, the semiconductor memory device 10 is a dynamic random access memory (DRAM). However, the invention can be incorporated into other semiconductor memory devices. The basic memory device 10 is well known in the art to include a memory array 12, address decoder circuits 14, read/write control circuits 16 and input/output circuits 18. The memory device 10 additionally includes a test mode circuit 20 for enabling a plurality of tests to be conducted on the memory device. It will be appreciated that the memory has been simplified to focus on the features of the present invention, and all the features of the memory have not been shown. A microprocessor, or memory controller can be coupled to the memory for providing control signals, address signals and bi-directional data communication signals.

The test mode circuit allows the memory to be evaluated using a wide variety of tests. These tests can be conducted during fabrication of the memory, or after the memory has been completed. The test mode can be initiated using any known techniques, including, but not limited to, an electronic key, supervoltages, specific combinations of control and address signals, or probe points. While an almost unlimited number of tests are available, testing of memory cell margins are particularly benefited by the present invention, as explained in greater detail below.

Referring to FIG. 2, the memory array 12 is constructed of a plurality of memory cells 22 arranged in a matrix and having inputs and outputs corresponding to the rows and the columns of the array. The array has differential paired digit (data) lines BL0, BL0* . . . BLN, BLN* and word (address) lines WL0, WL1 . . . WLN−1, WLN. The paired digit lines BL0, BL0* . . . BLN, BLN* are used to write information into the memory cell and to read data stored in the memory cell. Each of the paired digit lines includes a true state digit line, such as digit line BL0, and a complementary digit line, such as digit line BL0*. The word lines WL0, WL1 . . . WLN-1, WLN are used to address or select the memory cell to which data is to be written or read.

The address decoder circuits 14 include row decoder circuits 24 which control word line drivers to access the memory cells of the memory array 12 in response to address signals A0-AN that are provided by an external controller, such as a microprocessor. The address decoder circuit 14 also includes bit line drivers and column decoder circuits for selecting the bit lines. The input/output circuits 18 and other circuits, such as buffers which control either synchronous or asynchronous data communication between the memory device 10 and external devices in the conventional manner.

A reference signal generating circuit 26 functions as a source of a pre-charge or equilibrating voltage AVC2 which can be applied to the digit lines by sense amplifiers 32. A separate equilibration circuit 28 is provided for each of the paired digit lines, such as paired digit lines BL0 and BL0*, and is connected between the associated paired digit lines to equilibrate the potentials of paired digit lines. The sense amplifiers 32 are connected to each of the paired digit lines, such as BL0 and BL0*. The sense amplifiers 32 can include n-sense and p-sense amplifier circuits, as is known to one skilled in the art.

Memory Cell and Operation

A DRAM memory cell or memory bit, as shown in FIG. 3, consists of one MOS transistor 10 and one storage capacitor 12—accordingly referred to as a one-transistor one-capacitor (1T1C) cell. The memory bit transistor operates as a switch, interposed between the memory bit capacitor and the digit line 14. The memory bit is capable of holding a single piece of binary information, as stored electric charge in the cell capacitor. Given a bias voltage of Vcc/2 on the capacitor's common node, a logic one level is represented by +Vcc/2 volts across the capacitor and a logic zero is represented by −Vcc/2 volts across the capacitor.

The digit line, as depicted in FIG. 3, consists of a conductive line connected to a multitude of memory bit transistors. Generally, either metal or silicided/polycided polysilicon forms the conductive line. Due to the large quantity of attached memory bits, its physical length, and proximity to other features, the digit line is very capacitive. For instance, a typical value for digit line capacitance on a 0.35 um process might be around 300 fF. Digit line capacitance is an important parameter since it dictates many other aspects of the design.

The memory bit transistor's gate terminal connects to a word line (Row line) 16. The word line, which connects to a multitude of memory bits, consists of an extended segment of the same polysilicon used to form the transistor's gate. The word line is physically orthogonal to the digit line. A memory array, shown in FIG. 4, is created by tiling a selected quantity of memory bits together such that memory bits along a given digit line do not share a common word line and such that memory bits along a common word line do not share a common digit line.

Referring to FIG. 4, assume that the capacitor has a logic one level (+Vcc/2) stored on it. The digit lines, labeled D0 and D0* are initially equilibrated at Vcc/2 volts. All word lines are initially at zero volts, which turns off the memory bit transistors. To read memory bit1, word line WL0 transitions to a voltage that is at least one transistor V_(th) above Vcc. This elevated word line voltage level is referred to as Vccp or Vpp. When the word line voltage exceeds one V_(th) above the digit line voltage (Vcc/2 in this example) and the memory bit transistor turns on, the memory bit capacitor will begin to discharge onto the digit line. Essentially, reading or accessing a DRAM cell results in charge sharing between the memory bit capacitor and the digit line capacitance. This sharing of charge causes the digit line voltage to either increase for a stored logic one or decrease for a stored logic zero. A differential voltage develops between the two digit lines. The magnitude of this signal voltage is a function of the memory bit capacitance (C memory bit), digit line capacitance (Cdigit), the memory bit's stored voltage prior to the access (Vcell), and any noise terms. For a design in which Vcell=1.65V, Cmemory bit=40 fF, Cdigit=300 fF, and V_(noise)=0, this yields a digit line change of V_(signal)=194 mV. FIG. 5 contains waveforms for the cell access operation just described.

After the cell access is complete, the sensing operation can commence. The reason for forming a digit line pair will now become apparent. FIG. 6 contains a schematic diagram for a simplified typical sense amplifier circuit. Note that it consists of a cross-coupled PMOS pair 18 and a cross-coupled NMOS pair 20. The NMOS pair or N-sense-amp common node is labeled NLAT* (for N-sense-amp LATch) in FIG. 5. Similarly, the P-sense-amp common node is labeled ACT (for ACTive pull-up). Initially, NLAT* is biased to Vcc/2 and ACT is biased to VSS or ground. Since the digit line pair D0 and D0* are both initially at Vcc/2 volts, the N-sense-amp transistors remain off due to zero Vgs potential. Similarly, both P-sense-amp transistors remain off due to their negative Vgs potential. As discussed in the preceding paragraph, a signal voltage develops between the digit line pair when the memory bit access occurs. While one digit line contains charge from the cell access, the other digit line serves as a reference for the sensing operation. The sense amplifier firing generally occurs sequentially rather than concurrently. The N-sense-amp fires first and the P-sense-amp second. The N-sense-amp is generally a better amplifier than the P-sense-amp because of the higher drive of NMOS transistors and better V_(th) matching. This provides for better sensing characteristics and lower probability of errors. FIG. 7 contains waveforms for the sensing operation. Dropping the NLAT* signal toward ground will fire the N-sense-amp. As the voltage between NLAT* and the digit lines approaches V_(th), the NMOS transistor, whose gate connection is to the higher voltage digit line, will begin to conduct. Conduction results in the discharge of the low voltage digit line toward the NLAT* voltage. Ultimately, NLAT* will reach ground, bringing the digit line with it. Note that the other NMOS transistor will not conduct since its gate voltage derives from the low voltage digit line, which is discharging toward ground.

Shortly after the N-sense-amp fires, ACT will be drive toward Vcc. This activates the P-sense-amp that operates in a complementary fashion to the N-sense-amp. With the low voltage digit line approaching ground, a strong signal exists to drive the appropriate PMOS transistor into conduction. This will charge the high voltage digit line toward ACT, ultimately reaching Vcc. Since the memory bit transistor remains on during sensing, the memory bit capacitor will charge to the NLAT* or ACT voltage level. The voltage, and hence charge, which the memory bit capacitor held prior to accessing will restore a full level—Vcc for a logic one and GND for a logic zero.

For a memory write operation, the paired digit lines are charged to represent the data to be written into the memory cell. The word line WL0 is activated for connecting the memory cell to the digit line BL0 to allow charge sharing between the digit line and the capacitor C of the memory cell. It will be appreciated the memory read/write operations have been described in a simplified manner and that such access operations include numerous additional steps known to those skilled in the art.

Variable Equilibrate Circuit

Referring to FIG. 8, the equilibration circuit 28 of the present invention can include n-channel transistors 41 and 42 which are connected in series between the complementary digit lines BL0 and BL0*, and a further n-channel transistor 43 that is connected between the complementary digit lines BL0 and BL0* and in parallel with transistors 41 and 42. The gate electrodes of transistors 41, 42 and 43 are commonly connected to receive an equilibration enabling signal EQA that is produced by the read/write control circuits 16. It is pointed out that the read/write control circuits 16 produce separate equilibrating signals EQA and EQB, with signal EQA being applied to the equilibrating circuit associated with subarray 33A and the equilibrating signal EQB being applied to the equilibrating circuit associated with subarray 33B.

The n-sense amplifier circuit 37 can be comprised of a pair of cross-coupled, n-channel transistors 52 and 53. The source electrodes of transistors 52 and 53 are connected to a pulldown transistor 54 which is activated by signal NLAT. The NLAT line is strobed high to sense data stored on a selected memory cell.

Similarly, each p-sense amplifier circuit, such as sense amplifier circuit 39, is comprised of a pair of cross-coupled, p-channel transistors 56 and 57. The source electrodes of the p-channel transistors 56 and 57 are connected to a common enable line RPL which is normally held to Vcc/2 and is strobed high to sense data stored on the memory cells.

The source electrodes of transistors 52 and 53 are also coupled to pull-up transistor 58. During test operations transistor 54 is activated to sense a differential voltage between the digit lines. After the memory cells are isolated from the digit lines, transistor 58 is activated to raise the voltage on the digit line. When the digit lines are equilibrated, therefore, the resultant equilibrate voltage is greater than the normal equilibrate voltage, such as Vcc/2.

FIG. 9 illustrates the timing of the operation of transistor 58 following a data read. At time T1, the digit lines have been equilibrated to Vcc/2. A word line signal is then raised to couple a memory cell to a digit line. A differential voltage develops across the digit lines when the memory cell is accessed, as illustrated. The sense amplifiers are activated (not shown) to drive the digit lines to opposite power supply potentials. Note that the word lines remains active at an elevated voltage to restore the data to the memory cell. After the data is restored, the word line is lowered to isolate the memory cell. The EQNEW signal transitions low to activate transistor 58. Thus, the low digit line is charged through transistor 58 an the n-sense amplifier. When the EQNEW signal returns to a high state, the equilibration circuitry is activated. The new equilibrate level, therefore, is about equal to the average of the two digit line voltages and is greater than the normal operational level of Vcc/2. The low pulse width of the EQNEW signal can be adjusted to control the level of charge provided to the low digit line. It will be appreciated b Those skilled in the art that the equilibrate voltage can be reduced in the same manner by lowering the high digit line prior to equilibration. By adjusting the equilibrate voltage level, cell margin can be tested. That is, by systematically raising the equilibrate voltage the point where a memory cell fails to provide enough digit line signal for accurate sense amplifier detection can be established.

Conclusion

A memory has been described which includes a bias circuit coupled to a sense amplifier for adjusting a digit line voltage prior to equilibration. The bias circuit is described as a pull-up transistor coupled to the n-sense amplifier circuit. A method has been described for testing cell margin in a DRAM by raising the equilibrate voltage until data stored in a memory cell is erroneously read. The equilibrate voltage is adjusted by controlling a pulse width of the pull-up transistors gate signal.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. An equilibration circuit comprising: a pair of transistors having a common gate and connected in series between a pair of complementary digital lines; an n-channel transistor having a gate coupled to the common gate, and the n-channel transistor is connected between the pair of complementary digital lines and in parallel with the pair of transistors; a read/write control circuit to provide an equilibration enablement signal, the equilibration enablement signal is coupled to the common gate; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 2. An equilibration circuit comprising: a pair of complementary digital lines; and three or more transistors having a common gate, and each of the three or more transistors is connected between the pair of complementary digital lines and capable of altering a voltage on each of the pair of complementary digital lines independently; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having an n-mos transistor and a p-mos transistor wherein the n-mos transistor is operable for pulling down the common source and the p-mos transistor operable for pulling up the common source.
 3. The equilibration circuit of claim 2, wherein the pair of transistors cross-coupled between the pair of complementary digital lines are n-channel transistors.
 4. An equilibration circuit comprising: a circuit operable for equilibrating a pair of complementary digital lines; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 5. The equilibration circuit of 4, wherein the equilibration enablement signal is produced by a read/write control circuit.
 6. The equilibration circuit of claims 4, wherein the first transistor is a n-mos transistor and the second transistor is a p-mos transistor.
 7. An equilibration circuit comprising: a pair of transistors having a common gate and connected in series between a pair of complementary digital lines; an n-channel transistor having a gate coupled to the common gate, and the n-channel transistor is connected between the pair of complementary digital lines and in parallel with the pair of transistors; an equilibration enablement signal coupled to the common gate; a pair of complementary digital lines coupled to the pair of transistors and the transistor; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 8. The equilibration circuit of claim 7, wherein each of the pair of transistors is an n-channel transistor.
 9. A computer system comprising: a processor; a memory board coupled to the processor; and a memory device coupled to the memory board, the memory device comprising: a memory cell; a pair of digit lines coupled to the memory cell; a separate equilibration circuit provided for the pair of digit lines; a pair of transistors cross-coupled between the pair of digit lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors including a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor is operable for pulling up the common source.
 10. The computer system of claim 9, wherein the separate equilibration circuit is a transistor circuit.
 11. An equilibration circuit comprising: a pair of n-channel transistors having a common gate and connected in series between a pair of complementary digital lines; an n-channel transistor having a gate coupled to the common gate, and the n-channel transistor is connected between the pair of complementary digital lines and in parallel with the pair of n-channel transistors; a digital read/write control circuit to provide an equilibration enablement signal, the equilibration enablement signal is coupled to the common gate; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 12. An equilibration circuit comprising: a pair of complementary digital lines; and three or more n-channel transistors having a common gate, and each of the three or more n-channel transistors is connected between the pair of complementary digital lines and capable of altering a voltage on each of the pair of complementary digital lines independently; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having an n-mos transistor and a p-mos transistor wherein the n-mos transistor is operable for pulling down the common source and the p-mos transistor operable for pulling up the common source.
 13. The equilibration circuit of claim 12, wherein the pair of transistors cross-coupled between the pair of complementary digital lines are n-channel transistors.
 14. An equilibration circuit comprising: a circuit operable for equilibrating a pair of complementary digital lines; a pair of n-channel transistors cross-coupled between the pair of complementary digital lines, the pair of n-channel transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 15. The equilibration circuit of 14, wherein the equilibration enablement signal is produced by a digital read/write control circuit.
 16. The equilibration circuit of claim 14, wherein the first transistor is a n-mos transistor and the second transistor is a p-mos transistor.
 17. An equilibration circuit comprising: a pair of n-channel transistors having a common gate and connected in series between a pair of complementary digital lines; an n-channel transistor having a gate coupled to the common gate, and the n-channel transistor is connected between the pair of complementary digital lines and in parallel with the pair of n-channel transistors; an equilibration enablement signal coupled to the common gate; a pair of complementary digital lines coupled to the pair of n-channel transistors and the transistor; a pair of transistors cross-coupled between the pair of complementary digital lines, the pair of transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors having a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor operable for pulling up the common source.
 18. The equilibration circuit of claim 17, wherein each transistor in the pair of n-channel transistors is an metal-oxide-semiconductor n-channel transistor.
 19. A computer system comprising: a processor; a memory board coupled to the processor; and a memory device coupled to the memory board, the memory device comprising: a memory cell; a pair of digit lines coupled to the memory cell; a separate metal-oxide-semiconductor equilibration circuit provided for the pair of digit lines; a pair of n-channel transistors cross-coupled between the pair of digit lines, the pair of n-channel transistors having a common source; and a second pair of transistors coupled to the common source, the second pair of transistors including a first transistor and a second transistor wherein the first transistor is operable for pulling down the common source and the second transistor is operable for pulling up the common source.
 20. The computer system of claim 19, wherein the separate equilibration circuit is a metal-oxide-semiconductor transistor circuit.
 21. The computer system of claim 20, wherein the second pair of transistors comprise n-channel transistors.
 22. The computer system of claim 21, wherein the n-channel transistors comprise metal-oxide-semiconductor transistors.
 23. The computer system of claim 19, wherein the memory cell is a dynamic random access memory cell.
 24. The computer system of claim 19, wherein the second pair of transistors comprise n-channel transistors. 